bibliostone, I always just use my rulers for cutting. The only time I use the mat is to get a square edge to start. My 12 1/2" rotating mat matches my rulers perfecting. When the block is done I put it on that, line it up and square it up. When I cut out fabric I make sure the "line" on my ruler is "over", like on top of the fabric also, not beside it. You could be losing about a 16th of an inch and in a 12 1/2" block could amount to 1/4" if that's what you are short. Check this out...it may make a difference between your ruler and cutting mat.